The present invention relates generally to fault-protection systems for machines where rotor flux cannot be controlled, and more particularly, to fault protection systems for synchronous permanent magnet machines.
Permanent magnet machine (PMM) motors and generators are well known in the art. Such devices have windings that cut through the magnetic lines of force of a set of permanent magnets. The magnets may be arranged around a rotating set of windings or, conversely, windings may be arranged around a rotating magnet.
PMMs are relatively inexpensive and easy to construct and outperform other designs in most applications. A drawback, however, is that the rotor flux in a PMM is fixed and cannot be controlled or disengaged when a short circuit occurs. Unlike other designs, such as wound-field (WF), induction, and switch-reluctance machines (SRM), where the excitation of the rotor flux may be controlled or shut down completely, a PMM continues to generate voltage until the rotor stops spinning. The result is that PMMs typically suffer major damage when a short circuit occurs in the system, resulting from massive currents flowing through the windings and generating high temperatures that typically melt and burn the machine's components. Therefore, a PMM may present a hazard in some applications leading to its limited use, particularly in the aerospace industry.
There are a number of systems for short-circuit protection in the art. One such system is a high-reactance permanent magnet machine (HRPMM) that limits phase current magnitude upon a short circuit so as to limit heat and current damage. Another prior art system for short-circuit protection uses a dc contactor that isolates the load from the inverter so as to protect the load, but again fails to protect the PMM itself and the inverter/converter. Yet another prior art system uses a three-phase contactor in the feeder between the machine and an inverter, as shown in FIG. 1, which can protect the inverter from damage caused by a short in the machine, but not the machine itself.
Referring to FIG. 1, there is shown the prior art fault protection system 11 having a permanent magnet machine (PMM) 10 and a fault protection unit 26. The PMM 10 can be represented as an AC source 12 and an inductor 14 in series to simulate each winding 16, such as in an electric motor or generator. Each winding 16 is connected to the PMM's neutral point 20. An AC feeder 17 is provided, which may be little more than a set of wires that delivers the AC output of the PMM 10 to a load 24. If the load 24 is a DC device, then an inverter/converter 18 may be provided to rectify the PMM output to DC, which is then delivered to the load 24 via a DC feeder 22, which again may be nothing more than a set of wires. The prior art fault protection unit 26 is generally a set of switches 27 in the AC feeder 17 that respond to current sensors. If a short-circuit develops causing excessive current to flow through one of the windings 16, the sensor signals the PMM 10 to shut down and signals the switch 27 for that winding to open, thereby isolating the AC feeder 17, the inverter/converter 18, the DC feeder 22, and the load 24 from the short-circuited (or faulted) winding. If the short occurs downstream of the prior art fault protector switches 27, then the faulted PMM winding may be protected also. If the short occurs upstream of the prior art fault protection switches 27, then the PMM 10 may be damaged during the time it takes for the rotor to slow to a stop, typically about 10 seconds.
U.S. Pat. No. 6,577,086 discloses a method of machine neutral decoupling (MND) where the connection between the shorted winding and the machine neutral is broken. The '086 patent interposes a metal-oxide semiconductor field-effect transistor (MOSFET) between each winding and the machine neutral. The drawback with this design is that it is unidirectional in that current is blocked in only one direction. This necessitates the connection of a diode across each MOSFET that permits reverse current to flow from source to drain, so even when the MOSFET is shut off current still flows in the reverse direction. Hence, opening the MOSFET doesn't completely shut off the current in its associated winding, but merely reduces it to a half-wave. This may be sufficient for small electric motors, but for PMMs operating at high currents, this may not be sufficient to prevent damage.
Alternatively, U.S. Pat. No. 6,750,576 discloses a method for mitigating damage by a short-circuit by distributing the short-circuit current over a larger number of turns in the associated winding. The method of the '576 patent accomplishes this by, upon detection of a short-circuit in a turn or turn section of a winding, the entire coil is short-circuited at the terminals, resulting in a short-circuit current throughout the entire winding. The result is a reduction of the short-circuit current in the originally affected turn is reduced considerably so that the losses cause by the “distributed” short-circuit current can be compensated by coolants. However, like the other prior art systems described above, the fault current is only mitigated.
What is needed is a system for rapidly interrupting short-circuit current in a machine, thereby protecting not only the motor itself, but all the downstream components, such as an inverter and load. It would also be advantageous to have a system that can prevent the occurrence of a short circuit.